Results

Water Quality and Toxicity Testing

Water quality characteristics of ASTM very hard water were measured prior to
use in toxicity tests (Table 1) and were within acceptable guidelines (ASTM
1989). Throughout the experiment, dissolved oxygen concentrations were maintained
between 20% - 60% saturation before renewal and 50% - 70% saturation after renewal.

Measured concentrations of atrazine in the three exposure treatments were
0.23 ± 0.21 µg/L in the 0 µg/L treatment, 81.83 + 3.77 µg/L
in the 75 µg/L treatment, and 299.71 + 7.32 µg/L in the 250 µg/L
treatment. There were no exposure-related mortalities.

Table 1: Summary of water-quality characteristics
of blended water used
for exposure of tiger salamander larvae to atrazine (n = 6 grab samples).

Figure 1. Plasma corticosterone (a) and plasma thyroxine
(b) concentrations in stage 2 (open bars) and stage 4
(filled bars) larvae exposed to 0, 75, and 250 µg/L atrazine.
Shown are least squares means + 1 SE of the mean. We found a significant
treatment-by-stage interaction in which corticosterone concentrations
increased between stages 2 and 4 in both the control larvae and the
larvae exposed to 250 µg/L atrazine but not in those exposed to
75 µg/L atrazine. Plasma thyroxine concentration increased significantly
between stages 2 and 4 in all treatments and was significantly higher
at stage 4 in the two atrazine treatments compared with the control.

Growth and Timing

For each aspect of metamorphic timing and growth (days-to-stage, weight, and
snout-vent length), we found significant differences with respect to stage and
a significant stage-by-atrazine treatment interaction (Table 2). Preplanned
contrasts indicated that larvae in the 250 µg/L treatment took longer to
reach stage 2 (P = 0.04), and were smaller (P = 0.0007) and weighed
less (P = 0.006; Fig. 2) at stage 4 than larvae in the control treatment.
Larvae in the 75 µg/L treatment were marginally smaller at stage 2 (P
= 0.056) and reached stage 4 later (P = 0.05; Figure 2) than larvae in
the control group. Larvae in the 250 µg/L treatment reached stage 4 sooner
(P = 0.01), but were smaller (P = 0.01) and weighed less (P
= 0.004; Fig. 2) at stage 4 than larvae in the 75 µg/L treatment.